1. Field of the Invention
The invention relates to optical disk drives, and more particularly to feeding devices of optical disk drives.
2. Description of the Related Art
When an optical disk drive reads data from an optical disk with a high data density such as a blu-ray disk or a HD-DVD, a wavelength of a laserbeam emitted by a pickup head of the optical disk drive for reading data from the optical disk is reduced, and a numerical aperture of an objective lens for focusing the laserbeam on the optical disk is also increased. Increase of the numerical aperture increases probability of occurrence of spherical aberration which reduces precision of the signal reflected from a disk surface and increases error rate of data decoding. The optical disk drive therefore must comprise a mechanism for compensating for spherical aberration when the optical disk with a high data density is read.
A conventional mechanism for spherical aberration correction is a feeding device. A feeding device ordinarily comprises a spherical aberration (SA) lens and a stepping motor. The SA lens is positioned in a projecting path of the laserbeam to correct the spherical aberration. The stepping motor moves the SA lens to adjust a distance between an objective lens and the SA lens, thus allowing the SA lens to be put in an optimal position for precisely correcting the spherical aberration. Thus, precision of controlling the feeding device determines performance of spherical aberration correction.
A stepping motor is ordinarily controlled by a plurality of control signals generated by a power driver. Referring to FIG. 1, a table of an excitation pattern of control signals for driving rotation of a stepping motor is shown. Four control signals +A, −A, +B, and −B are shown in the table. When the control signals (+A, +B) applied to a stepping motor are in the sequential excitation pattern of (H, H), (H, L), (L, L), and (L, H), the stepping motor rotates clockwise. When the control signals (+A, +B) applied to the stepping motor are in the sequential excitation pattern of (L, H), (L, L), (H, L), and (H, H), the stepping motor rotates counterclockwise. Each phase change of the control signals triggers the stepping motor to rotate by a predetermined angle.
When the control signals +A and +B are in the phases of (H, H) or (L, L), the stepping motor is in a stable state, which may mean surrounding electromagnets and a central rotation gear of the stepping motor are in complementary magnetic directions for some type of stepping motor. When the control signals +A and +B are in the phases of (H, L) or (L, H), the stepping motor is in an instable state, for example, in which surrounding electromagnets and a central rotation gear of the stepping motor are in opposite magnetic directions. The stepping motor therefore cannot be held at an instable state for a long period. Thus, when the control signals have an instable phase such as (H, L) or (L, H) at a last step of a driving excitation, the stepping motor automatically rotates by an extra angle to enter a stable state. The automatically rotated extra angle is referred to as “step errors”. In addition, when the stepping motor is excited by the control signals for multiple times, the step errors accumulated with time become greater and greater, inducing inaccuracy in the position of the SA lens and requiring correction.
Referring to FIG. 2, a schematic diagram of a conventional method for correcting step errors is shown. Two driving excitations of the stepping motor are shown in FIG. 2. A first driving excitation comprises steps 1˜7 to drive clockwise rotation of the stepping motor. A second driving excitation comprises steps 9˜15 to drive counterclockwise rotation of the stepping motor. At a last step with a step number 7 of the first driving excitation, the control signals +A and +B have instable phases of (H, L) and step errors are therefore induced. To correct step errors, an initial excitation step with a step number 9 is added to the second driving excitation as a first step. The control signals +A and +B at the initial excitation step have the same phases (H, L) as that of the control signals at the latest step with the step number 7 in the prior driving excitation. Thus, step error is corrected, and the location of the SA lens is precisely controlled without being degraded due to step error accumulation.
The conventional method for step error correction, however, still has defects. Every time before a stepping motor is rotated, an initial excitation with a period of 40 ms is required for step error correction. Rotation of the stepping motor and motion of the SA lens are therefore delayed. When the SA lens is frequently required to move for spherical aberration correction, motion of the SA lens is frequently delayed, inducing delay in reading operation of the whole system and degrading system performance. Thus, a method for driving a feeding device of an optical disk drive without delays induced by initial excitation for step error correction is required.